Photovoltaics is the technology that typically transforms radiation, generally illumination, into electricity. Thermophotovoltaics is the technology of transforming infrared radiation, such as radiant heat generated by a source of energy, into electricity. Thermophotovoltaics offers a major advantage in certain settings as it can be operated at night or when the sky is overcast, thereby eliminating the need for batteries to store electricity. A semiconductor type thermovoltaic device can be designed to minimize pollutants, and, since it has no moving parts, will operate silently and reliably, requiring little maintenance.
TPV semiconductor devices have been heretofore made comprising a body of a suitable semiconductor material having a region of p-type conductivity adjacent a region of n-type conductivity forming a p-n junction therebetween. Since the material on both sides of the p-n junction has been the same, the p-n junction is a homojunction. To achieve direct conversion of black body (thermal) radiation to electrical energy, materials having a narrow band gap (E.sub.g) have been used to provide efficient absorption of the black body infrared radiation. For example, given a black body at a temperature of 1000.degree. C., materials with E.sub.g no greater than about 0.55 eV are desirable. In.sub.x Ga.sub.1-x As compound with x=0.7-0.75 grown on InP substrates have been successfully used for TPV applications. However, the fact that In.sub.0.7 Ga.sub.0.3 As layers are lattice mismatched with InP substrates limits the device performance. Recently, In.sub.0.16 Ga.sub.0.84 As.sub.0.05 Sb.sub.0.85 layers lattice matched to GaSb substrates have been used for TPV cells and the results are comparable to the best achieved for InGaAs/InP devices. Both of these types of TPV cells display internal quantum efficiencies higher than .sup.80 % for radiation from 1 micron to 2.1 micron. Thus, their potential for improvement of the internal quantum efficiency from these materials is rather small. However, the open circuit voltage for TPV cells designed for 1000.degree. C. black body radiation is only about 55% of E.sub.g for the 0.55 eV material band gap so that improvement in the open circuit voltage for TPV cells is desirable.